Laser printers are used in the photographic and graphic arts for printing on a photosensitive medium such as film or paper. When such printers are used for color imaging, they generally include three different light sources, each providing a light beam of a different wavelength. Light beams from these light sources are individually shaped by three separate sets of beam shaping optics (one set for each light beam), and are then combined into a single light beam with three coaxial components. This light beam is then scanned over an angular range by a rotating polygon with a plurality of reflective facets. An f-.theta. lens is located in the optical path between the polygon and the photosensitive medium. The single light beam, when incident on the polygon and the f-.theta. lens, would be typically collimated in each of the three wavelengths. Such printer is disclosed in U.S. Pat. No. 4,821,113. Not only does this printer have a separate set of beam shaping optics for each light beam, but it also operates in the infrared (I.R.) light region.
Because the printer is operating in I.R. region, this printer can not use conventional, silver halide based, photographic paper for printing images. Instead it has to use "false sensitized" paper that is matched to specific wavelengths of interest (i.e., 780 nm, 830 nm and 880 nm). More specifically, the conventional photographic paper is designed to be exposed with visible light and incorporates blue, green and red sensitive layers that produce yellow, magenta, and cyan dye layers in the final image. Laser printers designed to write with lasers that operate in the infrared region require, for their photosensitive medium, a special "false-sensitized" paper in which the red, green and blue sensitive layers are replaced by layers which are sensitive to three distinct infrared wavelengths. Development, production, and distribution of the "false-sensitized" paper is expensive.
U.S. Pat. Nos. 5,111,325 and 5,018,805 each disclose a color printer that operates with light beams at the wavelengths 750 nm, 810 nm, and 870 nm (I.R. region). This printer also utilizes three sets of beam shaping optics. More specifically, the f-.theta. lens disclosed in U.S. Pat. No. 5,111,325 operates with three light beams whose vergences are controlled independently of one another. The word "vergence", as used herein refers to whether the input light beam to the f-.theta. lens is collimated, converging or diverging. A collimated light beam has zero vergence, a diverging light beam has negative vergence and a converging light beam has positive vergence. The concept of "vergence" is described in more detail in column 4, lines 46-62 of the above mentioned U.S. Pat. No. 5,111,325. Since axial color is a focus related phenomenon, it is caused not only by aberrations of the f-.theta. lens itself, but also by the vergence of the input beam to the f-.theta. lens. In the printer disclosed in U.S. Pat. No. 5,111,325 and U.S. Pat. No. 5,018,805, the desired vergence in each of the three light beams is controlled by its own set of beam shaping optics located in each color channel. Therefore, at least three sets of independent beam shaping optics are required by this printer. These three sets of beam shaping optics provide the ability to control the vergences of the three light beams separately, which greatly reduces the problem of correcting axial color, but increases complexity and the cost of manufacturing and assembly.
In U.S. Pat. No. 4,728,965 there is disclosed a color printer having three light sources, each providing a visible wavelength light beam (red, green, blue, respectively). This printer can operate with photographic paper. However, this printer also has three sets of beam shaping optics (one for each channel).
In U.S. Pat. No. 5,475,415, there is disclosed a thermal printer with multiple light sources. In this type of printer, all of the light beams have the same wavelength. The multiple light sources are simply used to increase the throughput of the printer. Because all of the light beams have the same characteristics (wavelength and divergence) they can be shaped by a single set of beam shaping optics. However, such a configuration is not suitable for color printers using light beams with different wavelengths and different vergences because of chromatic aberrations.